The present invention relates to a technology for developing a seat belt retractor to be installed in a vehicle.
Conventionally, a seat belt apparatus which is designed for protecting and restraining a vehicle occupant by a seat belt (webbing) is known. For example, Japanese translation of PCT international application No. 2003-507252 (incorporated by reference herein) discloses a seat belt apparatus having a seat belt retractor with a spool or winding shaft, which is driven by an electric motor to rotate so as to wind or unwind a seat belt.
The seat belt retractor disclosed in PCT international application No. 2003-507252 is a structure in which the action of winding the seat belt onto the spool is conducted by the electric motor. However, when this structure is used for winding up a seat belt for the purpose of preventing the seat belt from being kept in an unwound state, i.e., a “seat belt storage winding action,” the action of winding up the seat belt onto the spool is required to be smoothly conducted according to a suitable control for the electric motor.
Specifically, during the seat belt storage winding action, the seat belt may be caught on a vehicle occupant or a vehicle seat or the seat belt may be withdrawn by the vehicle occupant. There is a solution to this situation, which is, when the load of the seat belt is increased (by detecting an increase in the motor current) during the seat belt storage winding action, a control to temporarily stop the seat belt storage winding action is engaged until the factor increasing the load is removed and then the seat belt storage winding action is restarted. However, when the seat belt is caught on the vehicle occupant or the vehicle seat, not much time is required to release the caught seat belt. In fact, if the seat belt storage winding action is stopped for a time longer than necessary, there is a possibility that the seat belt or the tongue is jammed in a vehicle door. On the other hand, when the seat belt is withdrawn by the vehicle occupant, a longer time period may be required for the seat belt withdrawing operation by the vehicle occupant. Therefore, if the seat belt storage winding action is stopped only for a short time period, the seat belt storage winding action operates against the seat belt withdrawing operation. Thus, there is a problem that the vehicle occupant may be made to feel uncomfortable. To streamline the seat belt storage winding action, it may be required to temporarily stop the seat belt storage winding action and to set the time interval of stoppage to a suitable time commensurate with the factor causing the increase to the seat belt load.